15.8 Setting Site Defaults

Autoconf-generated configure scripts allow your site to provide
default values for some configuration values. You do this by creating
site- and system-wide initialization files.

If the environment variable CONFIG_SITE is set, configure
uses its value as the name of a shell script to read; it is recommended
that this be an absolute file name. Otherwise, it
reads the shell script prefix/share/config.site if it exists,
then prefix/etc/config.site if it exists. Thus,
settings in machine-specific files override those in machine-independent
ones in case of conflict.

Site files can be arbitrary shell scripts, but only certain kinds of
code are really appropriate to be in them. Because configure
reads any cache file after it has read any site files, a site file can
define a default cache file to be shared between all Autoconf-generated
configure scripts run on that system (see Cache Files). If
you set a default cache file in a site file, it is a good idea to also
set the output variable CC in that site file, because the cache
file is only valid for a particular compiler, but many systems have
several available.

You can examine or override the value set by a command line option to
configure in a site file; options set shell variables that have
the same names as the options, with any dashes turned into underscores.
The exceptions are that --without- and --disable- options
are like giving the corresponding --with- or --enable-
option and the value ‘no’. Thus, --cache-file=localcache
sets the variable cache_file to the value ‘localcache’;
--enable-warnings=no or --disable-warnings sets the variable
enable_warnings to the value ‘no’; --prefix=/usr sets the
variable prefix to the value ‘/usr’; etc.

Site files are also good places to set default values for other output
variables, such as CFLAGS, if you need to give them non-default
values: anything you would normally do, repetitively, on the command
line. If you use non-default values for prefix or
exec_prefix (wherever you locate the site file), you can set them
in the site file if you specify it with the CONFIG_SITE
environment variable.

You can set some cache values in the site file itself. Doing this is
useful if you are cross-compiling, where it is impossible to check features
that require running a test program. You could “prime the cache” by
setting those values correctly for that system in
prefix/etc/config.site. To find out the names of the cache
variables you need to set, see the documentation of the respective
Autoconf macro. If the variables or their semantics are undocumented,
you may need to look for shell variables with ‘_cv_’ in their names
in the affected configure scripts, or in the Autoconf M4
source code for those macros; but in that case, their name or semantics
may change in a future Autoconf version.

The cache file is careful to not override any variables set in the site
files. Similarly, you should not override command-line options in the
site files. Your code should check that variables such as prefix
and cache_file have their default values (as set near the top of
configure) before changing them.

Here is a sample file /usr/share/local/gnu/share/config.site. The
command ‘configure --prefix=/usr/share/local/gnu’ would read this
file (if CONFIG_SITE is not set to a different file).

Another use of config.site is for priming the directory variables
in a manner consistent with the Filesystem Hierarchy Standard
(FHS). Once the following file is installed at
/usr/share/config.site, a user can execute simply
./configure --prefix=/usr to get all the directories chosen in
the locations recommended by FHS.